Also, if you’re using dynamic image resizing, you may want to disable this if possible.

If all else fails, contact the theme provider for assistance, or change to a different theme.

Disable CPU intensive plugins, or plugins that you no longer need.

All related posts plugins (WordPress Related Posts, YARPP) can cause significantly high load in most cases.

WPRobot3 and other auto-posters can also cause high load issues, and should be disabled if they are causing issues

StatPress and other wordpress statistics software should also be disabled, as these too can consume too much CPU in certain cases. Use Google Analytics instead for statistics.

Any other plugins that are not vital to your WordPress should be disabled.

Manage Spam bots. Non-human visitors/posters (aside from web spiders) should be blocked. Use a captcha such as SI CAPTCHA to prevent comment spam.

Lower the rate web spiders crawl your WordPress blogs, if you have a large amount of blogs hosted under your account. High crawl rates can drive up the load on the server, as many bots try to index your sites.

These changes are not guaranteed fixes, but do provide a starting ground for correcting high load issues on WordPress sites.